Semiconductor component for transient voltage limiting

ABSTRACT

A semiconductor component for limiting transient voltages on the signal or other supply lines of a system, includes, in a common semiconductor body, a plurality of multi-junction diodes connected in the same sense between a common terminal and respective input means which are for connection to the respective supply lines of the system, and a respective further diode connected in shunt with each multi-junction diode with the opposite sense to the multi-junction diode.

The present invention relates to a semiconductor component for limitingtransient voltages on supply lines.

The semiconductor component is especially suitable for protectingtelephone system components from transient disturbances which wouldappear on the signal lines of the telephone system.

Past telephone systems used electromechanical components which couldwithstand high transient voltages which occur on signal lines as aresult of natural electrical disturbances such as lightning strikes, forexample. The components used in modern telephone systems consist mainlyof semiconductor devices which are vulnerable to the high transientvoltages which occur in telephone systems, so there is a requirement fora protective device, or protective devices, for telephone systemcomponents.

Semiconductor devices have also replaced more robust devices in othersituations where they may be subjected to, and need to be protectedfrom, high transient voltages, so there is a requirement for aprotective device, or protective devices, for semiconductor devices insystems other than telephone systems.

It is an object of the present invention to provide a semiconductorcomponent, which, when connected to the signal or other lines of asystem, is capable of limiting transient voltages induced in thoselines.

In accordance with the present invention, a semiconductor component,suitable for limiting transient voltages on the supply lines of a systemhaving at least three supply lines, includes at least three input meansfor connection to respective ones of the supply lines, and, for eachinput means, a respective multi-junction diode which has a thresholdvoltage at which it changes from a high-impedance state to alow-impedance state, each multi-junction diode being connected in thesame sense between a respective input means and a common terminal, andeach multi-junction diode being paired with a respective further diodeconnected in shunt with it and in the opposite sense to it and one pairof the diodes having current capacities substantially equal to thecombined current capacities of the other pairs of diodes.

The form of the semiconductor component having three input means isespecially suitable for use in a telephone system.

Preferably, the cathode electrodes of the multi-junction diodes areconnected to respective input means.

In a first form of the semiconductor component, each of the furtherdiodes is a multi-junction diode.

In a second form of the semiconductor component, each of the furtherdiodes is a diode having a single PN junction.

In a further form of the semiconductor component, at least one, but notall, of the further diodes is a multi-junction diode, and each of theother further diodes has a single PN junction.

Preferably, the semiconductor component has the form of a semiconductorbody having two substantially parallel major surfaces, wherein theelectrodes of the diodes lie on the major surfaces, the input means area plurality of electrical conductors which make contact, on one of themajor surfaces, with the electrodes of respective pairs of diodes, afurther electrical conductor which is the common terminal makes contacton the other of the major surfaces with all of the electrodes at thatsurface, and the other pairs of diodes are symmetrically positioned inrelation to the pair of diodes having current capacities substantiallyequal to the combined current capacities of the other pairs of diodes.

Preferably, the diodes are alternately a multi-junction diode and afurther diode, both along and across the major surface, and each inputmeans contacts a diode pair comprising a multi-junction diode and afurther diode, and, preferably, each input means extends across a partof the major surface.

In one arrangement for the semiconductor component, each of the furtherdiodes is a multi-junction diode and an isolation region isolates eachdiode pair electrically from the other multi-junction diode pairs whichoccupy the same semiconductor body.

In an alternative arrangement for the semiconductor component, each ofthe further diodes has a single PN junction and a doped region of thesemiconductor body is common to all the diode pairs.

In an especially fast-acting alternative arrangement for thesemiconductor component, the multi-junction diodes are grouped aroundthe middle of the semiconductor body, and a doped region of thesemiconductor body is common to all the diode pairs.

Preferably, each multi-junction diode includes a cathode (n⁺) regionwhich has a plurality of gaps which are filled by the material of theadjacent p-type gate region.

The penetration of the cathode region by the adjacent p-type regionresults in the cathode-gate junction being shunted by a part of thep-type gate region. That arrangement increases the current required tomaintain the component in a conductive state compared with a componentwith an uninterrupted cathode region, and the designer of the componentis able to set the holding current by the layout of the penetratedcathode region.

In the second form of the semiconductor component, each multi-junctiondiode can have a substantially PNPN structure with a lightly dopedn-type (n⁻) inner N region, each further diode can have a heavily dopedn-type (n⁺) region, a lightly doped n-type (n⁻) region adjacent to then+ region, and a p-type region adjacent to the n⁺ region, and the n-typeinner N region of the multi-junction diode can be thicker than the n⁺region of the further diode, where thickness is measured in thedirection of current flow through the semiconductor component.

Preferably, each multi-junction diode has a substantially PNPNstructure, and includes, in the inner N region which is a lightly dopedn-type (n⁻) region, a buried n-type region of greater impurityconcentration than the n⁻ -region, wherein the buried n-type region isadjacent to the inner P region of the PNPN structure.

The second form of the semiconductor component in the form of asemiconductor body is so structured that, when one of the first pair ofdiodes conducts, its electrical charge is free to flow to and initiateconduction of at least one of another pair of diodes.

Advantageously, the multi-junction diodes are grouped around the middleof the semiconductor body and, advantageously, the multi-junction diodesare so structured as to conduct charge more readily near the middle ofthe semiconductor body than elsewhere in the semiconductor body.

In a modified structure for the second form of the semiconductorcomponent, each multi-junction diode is so structured as to operate as afirst multi-junction diode near the middle of the semiconductor body andas a second multi-junction diode elsewhere in the semiconductor body,the first multi-junction diode being smaller and more sensitive than thesecond multi-junction diode.

In the following description, the preferred form of the multi-junctiondiode is referred to as a bulk breakdown diode. One form of bulkbreakdown diode is described in UK patent No. GB 2 113 907B.

Examples of semiconductor components for limiting transient voltages, inaccordance with the invention, will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 is a sectional perspective view of an integrated circuitarrangement of a first form of a semiconductor component, for limitingtransient voltage changes, suitable for telephone system protection,

FIG. 2 is a sectional perspective view of an integrated circuitarrangement of a second form of a semiconductor component, for limitingtransient voltage changes, suitable for telephone system protection,

FIG. 3 is a circuit diagram representation of the first form of thesemiconductor component showing the device as a plurality of bulkbreakdown diodes,

FIG. 4 is a circuit diagram representation of the second form of thesemiconductor component showing the component as a plurality of bulkbreakdown diodes and rectifier diodes,

FIG. 5 is a circuit diagram representation of either form of thesemiconductor component showing the component as a plurality of blocks,

FIG. 6 is a simplified structural representation of a single bulkbreakdown diode,

FIG. 7 is a development of the simplified structural representation ofthe single bulk breakdown diode,

FIG. 8 is a circuit diagram representation of the single bulk breakdowndiode, derived from FIG. 7,

FIG. 9 is a symbol representing the single bulk breakdown diode,

FIG. 10 is a simplified structural representation of a combined bulkbreakdown diode and reverse-connected rectifier diode,

FIG. 11 is a symbolic representation of the structure of FIG. 10,

FIG. 12 is a representation of the current-voltage characteristic of thecomponent represented by FIG. 10,

FIG. 13 is a representation of the current-voltage characteristic of apair of bulk breakdown diodes, which operate as devices connected inshunt with each other with opposing polarities,

FIG. 14 is a plan view of a monolithic integrated circuit form of acombined bulk breakdown diode and reverse-connected rectifier diode,

FIG. 15 is a sectional view of the monolithic integrated circuit shownin FIG. 14 taken along the line XX,

FIG. 16 is a plan view of a monolithic integrated circuit form of abidirectional bulk breakdown diode,

FIG. 17 is a sectional view of the monolithic integrated circuit shownin FIG. 16 taken along the line XX,

FIG. 18 is a plan view of an alternative integrated circuit arrangementof the second form of the semiconductor component and,

FIG. 19 is a sectional view of the monolithic integrated circuit shownin FIG. 18 taken along the line YY.

Referring to FIG. 1 of the accompanying drawings, the first form of asemiconductor component for limiting transient voltage changes includesa first pair of bulk breakdown diodes 1, a second pair of bulk breakdowndiodes 2, and a third pair of bulk breakdown diodes 3, which three pairsof bulk breakdown diodes 1,2, and 3, belong to a monolithic integratedcircuit 4, and, which, in the monolithic integrated circuit 4, areisolated from one another by a continuous barrier 5. The barrier 5 formsthe sides of the integrated circuit 4 in addition to extending betweenthe bulk breakdown diodes 1, 2, and 3.

Referring to FIG. 1, the first pair of bulk breakdown diodes 1 includesa first layer 10 of p-type semiconductor material and a second layer 11of n⁻ -type semiconductor material in contact with the first layer 10. Ap-type region 13 lies in the second layer 11 and has a common surfacewith the second layer 11. There is an n-type region 12 below the p-typeregion 13, in contact with the p-type region 13 and an n⁺ -type region14 in the p-type region 13. The n⁺ -type region 14 has a common surfacewith the second layer 11 and with the p-type region 13. The n⁺ -typeregion 14 is penetrated at a plurality of regularly spaced locations bythe p-type region 13. An n⁺ -type ring 15 in the second layer 11 has acommon surface with the second layer 11.

Referring to FIG. 1, the n⁺ -type region 14 occupies substantially halfthe length of the p-type region 13 and almost, but not quite, the fullwidth of that p-type region. The boundary of the n-type region 12 issubstantially in alignment with that of the n⁺ -type region 14.

Included in the first pair of bulk breakdown diodes 1, but not shown inFIG. 1, are a further n-type region, which lies in contact with thep-type region 10 and in the n⁻ -type region 11, and a further n⁺ -typeregion, which lies within the p-type region 10 and has a surface commonwith the lower surface (as seen in FIG. 1) of the p-type region 10. Thefurther regions occupy the lower rear (as seen in FIG. 1) of thestructure of the first pair of bulk breakdown diodes 1.

FIG. 17 is a sectional view of the structure of a pair of bulk breakdowndiodes, such as the pair of bulk breakdown diodes 1 of FIG. 1, takenalong a line from front to rear (as viewed in FIG. 1), where the n-typeregion 16, shown in FIG. 17, is the further n-type region describedabove, and the n⁺ -type region 17 is the further n⁺ -type regiondescribed above.

The pair of bulk breakdown diodes represented by FIG. 17 includeselectrical contacts 19 and 110 on the top and bottom (as viewed in FIG.16), respectively, whereas FIG. 1 does not include the electricalcontacts for the device.

Referring to FIG. 17, the penetration of the n⁺ -type region 14 by theadjacent p-type region 13 results in the n⁺ -type region 14 beingshunted by those parts of the p-type region 13 which penetrate it. Theshunting effect of the p-type region 13 is used to set, for the bulkbreakdown diode, the current at which it changes its impedance state,that current being called its holding current. The bulk breakdown diodehas a low impedance when conducting currents higher than its holdingcurrent and switches to a high impedance state should its current fallbelow the holding current. The facility for setting the holding currentof the bulk breakdown diode is used to ensure that the bulk breakdowndiode switches to its high impedance state at a current which is abovethe short-circuit current for the system to which it is connected,allowing a return to the normal operation of the system once thetransient voltage has passed. The holding current of the bulk breakdowndiode is smallest when there is no penetration of the n⁺ -type region 14by the p-type region 13, and increases with the proportion of the n⁺-type region which is penetrated. The bulk breakdown diode whichincludes the regions 10, 11, 12, 13 and 14 performs as a "crowbar"switch, as described above, with its p-type region 10 positive withrespect to its n⁺ -type region 14. Without the n-type region 12, thediode would have an intrinsic breakdown voltage set by the n⁻ -typeregion 11, and the ion-implanted n-region 12 sets a breakdown voltagelower than the intrinsic breakdown voltage. The addition of theion-implanted n-region 12 permits accurate setting of the bulk breakdowndiode triggering (or "crowbar") voltage and, if required the fabricationof pairs of bulk breakdown diodes with different triggering voltages.That is, the ion-implanted n-type region 16 need not be identical to then-type region 12.

Referring to FIG. 1, the third bulk breakdown diode pair 3 issubstantially the same as the first bulk breakdown diode 1 in structureand its orientation, while the second bulk breakdown diode pair 2 isreversed in orientation relative to the other two bulk breakdown diodepairs 1 and 2, and is of substantially the same construction.

In the component represented by FIG. 1, an electrical contact isrequired along the lower surface (as viewed in FIG. 1) common to thethree pairs of bulk breakdown diodes 1, 2, and 3, and respectiveelectrical contacts are required at the upper surfaces (as viewed inFIG. 1) of the pairs of bulk breakdown diodes 1, 2, and 3.

When the component represented by FIG. 1 is provided with electricalcontacts, it may be represented by the devices shown in the circuit ofFIG. 3, in which the pairs of bulk breakdown diodes 1, 2, and 3 have acommon connection 6, and the respective pairs of bulk breakdown diodeshave free terminals 7, 8, and 9. The free terminals 7, 8, and 9 of FIG.3 are the separate electrical contacts for the respective pairs of bulkbreakdown diodes of FIG. 1, and the common connection 6 is the commonelectrical contact for the three pairs of bulk breakdown diodes.

In the operation of the integrated circuit, as represented by FIGS. 1and 3, a positive-going disturbance at the terminal 7, relative to theterminal 8, is limited to the sum of the triggering voltage of theforward biassed bulk breakdown diode of the bulk breakdown diode pair 1and the triggering voltage of the forward biassed bulk breakdown diodeof the bulk breakdown diode pair 2. A negative going voltage disturbanceat the terminal 7 relative to the terminal 8 is limited to the sum ofthe triggering voltages of the other two bulk breakdown diodes of thebulk breakdown diode pairs 1 and 2. Disturbing voltages at the terminal9, relative to the terminal 8, are limited, by the respective seriescombinations of the bulk breakdown diodes of the bulk breakdown diodepairs 2 and 3, in a manner similar to that described for disturbingvoltages at the terminal 7.

Referring to FIG. 5, the semiconductor component includes threeprotectors 2000, 2100 and 2200 each of which is required to withstandonly a proportion of the voltage which protectors connected in a deltaconfiguration would need to withstand. Because of the use of the starconfiguration, the component is implemented by the use of verticalstructures alone and has a higher current-handling capacity than knownprotection devices.

Referring to FIGS. 1 and 3, the first bulk breakdown diode pair 1, whenused in the protection of the components of a telephone system, has itsterminal 7 connected to the R or A line, its terminal 9 connected to theT or B terminal, and its terminal 8 connected to the ground line, of thetelephone system. In the operation of the integrated circuit asrepresented by FIGS. 1 and 3, transient voltage changes which occur onboth the R(or A) and T(or B) lines give rise to currents in both thebulk breakdown diode pairs 1 and 3, and the bulk breakdown diode 2 mustsink both currents, so the second bulk breakdown diode 2 requires twicethe current capacity of each of the other two bulk breakdown diodes 1and 3. The relative current capacity requirements of the bulk breakdowndiode pairs are met by making the areas of the active n⁺ -type regionsof the diodes of the second bulk breakdown diode pair 2 twice thecorresponding areas of the diodes of the other two bulk breakdown diodepairs 1 and 3.

In the operation of the integrated circuit as represented by FIGS. 1 and17, electrical charge generated in any one bulk breakdown diode pair isconfined to that bulk breakdown diode pair by the isolating barrier 5which surrounds each bulk breakdown diode pair.

Referring to FIG. 2 of the accompanying drawings, the second form of asemiconductor component, for limiting transient voltage changes,includes a first bulk breakdown diode-rectifier diode combination 200, asecond bulk breakdown diode-rectifier diode combination 210, and a thirdbulk breakdown diode-rectifier diode combination 220.

Referring to FIG. 2, the first bulk breakdown diode-rectifier diodecombination 200 includes a p-type region 201, an n⁻ -type region 202 incontact with the p-type region 201, an n-type region 203 which lieswithin the n⁻ -type region 202, a p-type region 204 which also lieswithin the n⁻ -type region 202, is in contact with the n-type region203, and has a common surface with the n⁻ -type region 202, and an n⁺-type region 205 which lies within the p-type region 204 and has acommon surface with the p-type region 204 and with the n⁻ -type region202. An n⁺ -type ring 206 in the n⁻ -type region 202 surrounds theregion 204 and has a common surface with that region.

Referring to FIG. 2, the first bulk breakdown diode-rectifiers diodecombination 200 includes an n⁺ -type region to the rear of the p-typeregion 201 (as viewed in FIG. 2). The n⁺ -type region to the rear of thep-type region 201 is not visible in FIG. 2, but is visible in FIG. 14,which represents a sectional view of the bulk breakdown diode-rectifierdiode combination 200 taken along a line running from its front to itsrear (as viewed in FIG. 2). The n⁺ -type region 207, shown in FIG. 14,would be the n⁺ -type region to the rear of the p-type region 201 inFIG. 2.

Referring to FIG. 2, the second bulk breakdown diode-rectifier diodecombination 210 has substantially the same structure as the first bulkbreakdown diode-rectifiers diode combination 200, but it is reversedrelative to the first bulk breakdown diode-rectifier diode combination200. As seen in FIG. 2, the second bulk breakdown diode-rectifier diodecombination 210 includes an n⁺ -type region 211, an n⁻ -type region 212in contact with the n⁺ -type region 211, a p-type region 213 lying inthe n⁻ -type region 212 and having a common surface with that of the n⁻-type region 212, and an n⁺ -type region 214 lying in the p-type region,to the rear of the p-type region 213 (as viewed in FIG. 2), and having acommon surface with the p-type region 213. The n⁺ -type region 211corresponds to the n⁺ -type region 207 of FIG. 14, and it will beunderstood that there are regions in the second bulk breakdowndiode-rectifier diode combination 210 corresponding to the regions 201and 203 of the first bulk breakdown diode-rectifier diode combination200.

Referring to FIG. 2, the third bulk breakdown diode-rectifiers diodecombination 220 is substantially the same as, and has the same relativeorientation as, the first bulk breakdown diode-rectifier diodecombination 200. The regions 221, 222, 223, 224, and 225 of the thirdbulk breakdown diode-rectifier diode combination 220 correspond to theregions 201. 202, 203, 204 and 205, respectively, of the first bulkbreakdown diode-rectifiers diode combination 200.

Referring to FIG. 2, the n⁺ -type ring 206 is a part of a ring systemwhich extends around each of the bulk breakdown diode-rectifier diodecombinations 200, 210, and 220. Contacts are required along the lowersurface of the integrated circuit, and a contact is required on theupper surface of each bulk breakdown diode-rectifier diode combination200, 210, and 220 (upper and lower referring to the integrated circuitas viewed in FIG. 2).

When the integrated circuit represented by FIG. 2 is provided withcontacts, it may be represented by the circuit shown in FIG. 4, in whichthe bulk breakdown diode-rectifier diode combinations 200, 210, and 220have a common connection 250 and terminals 251, 252, and 253.

Referring to FIG. 2, it is to be noted that the respective n⁻ -typeregions 202, 212, and 222 of the three bulk breakdown diode-rectifierdiode combinations 200, 210, and 220 are provided by a common n⁻ -typeregion. It is to be noted, also, that, at the front of the integratedcircuit (as viewed in FIG. 2), the rectifier diode of the centre bulkbreakdown diode-rectifier diode combination 210 lies between the bulkbreakdown diodes of the outer bulk breakdown diode-rectifier diodecombinations 200 and 220; at the rear of the integrated circuit (asviewed in FIG. 2), the rectifier diodes of the outer bulk breakdowndiode-rectifiers diode combinations 200 and 220 flank the bulk breakdowndiode of the centre bulk breakdown diode-rectifier diode combination210. Thus the centre bulk breakdown diode-rectifiers diode combination210 is of reversed orientation relative to the two outer bulk breakdowndiode-rectifiers diode combinations 200 and 220, and that reversal inorientation is responsible for the relationships between the adjacentcomponents of the integrated circuit changing from the front to the rearof the integrated circuit (as viewed in FIG. 2).

In the operation of the integrated circuit represented by FIGS. 2 and 4,a positive-going disturbance at the terminal 251, relative to theterminal 252, is limited to the sum of the forward breakdown voltage ofthe rectifier diode of the bulk breakdown diode-rectifier diode pair 200and the triggering voltage of the bulk breakdown diode of the bulkbreakdown diode-rectifier diode pair 210. A negative-going disturbanceat the terminal 251, relative to the terminal 252, is limited to the sumof the triggering voltage of the bulk breakdown diode of the bulkbreakdown diode-rectifier diode pair 200 and the forward breakdownvoltage of the rectifier diode of the bulk breakdown diode-rectifierdiode pair 210. Disturbing voltages at the terminal 253, relative to theterminal 252, are limited, by the components of the bulk breakdowndiode-rectifier diode pairs 220 and 210, in a manner similar to thatdescribed for disturbing voltages at the terminal 251.

In the operation of the integrated circuit represented by FIGS. 2 and 4,the bulk breakdown diodes of the bulk breakdown diode-rectifier diodepairs 200 and 220 are triggered at substantially the same time if thereis a common disturbing voltage at the terminals 251 and 253, and thatcommon disturbing voltage exceeds either of the voltage limits set forthose terminals. That result is achieved because the disturbance at theterminal 251, relative to the terminal 252, if it is negative and itexceeds the voltage allowed for a disturbance at that terminal, wouldcause the rectifier diode of the bulk breakdown diode-rectifier diodepair 210 and the bulk breakdown diode of the bulk breakdowndiode-rectifier diode pair 200 to conduct. As the rectifier diodeconducts it injects charge carriers into the n⁻ -type region 212, whichcharge carriers travel from the n⁻ -type region 212 into the n⁻ -typeregion 222. The presence of charge carriers in the region 222 cause thetriggering of the bulk breakdown diode of the third bulk breakdowndiode-rectifier diode pair 220. The triggering of the first bulkbreakdown diode-rectifier diode pair 200 and the third bulk breakdowndiode-rectifier diode pair 220 would occur at substantially the sametime, and, as a result, the disturbing voltages at the terminals 251 and253 would be removed at substantially the same time. If, alternatively,a common positive disturbing voltage appears on the terminals 251 and253, and the bulk breakdown diode of the bulk breakdown diode-rectifierdiode pair 210 triggers, the voltages at the terminals 251 and 253 wouldbe clamped by the conduction of the rectifier diodes of the bulkbreakdown diode-rectifier diode pairs 200 and 220. The rectifier diodeswould conduct at substantially the same time and would havesubstantially the same voltage drop.

Referring to FIG. 2, the substantially simultaneous triggering of morethan one of the diode pairs, in the situations described above, resultfrom three aspects of the structure of the component. The first aspectis that of so siting the bulk breakdown diodes as to allow chargecarriers from each of those bulk breakdown diodes, when triggered, toreach at least one other bulk breakdown diode through their common n⁻-type region 202-212-222. In that respect, the bulk breakdown diode ofthe central bulk breakdown diode-rectifier diode combination 210 is ableto supply charge carriers to, and to receive charge carriers from, eachof the other bulk breakdown diode-rectifiers diode combinations 200 and220, with the result that all the bulk breakdown diode-rectifier diodecombinations will be triggered once the central bulk breakdowndiode-rectifier diode combination 210 is triggered. The second aspect isthat of making the p-type regions represented by the regions 201 and 211in FIG. 2 common to the bulk breakdown diodes of the three diode pairsin order to achieve a current mirror effect, whereby the triggering ofany one bulk breakdown diode will result in the triggering of theadjacent bulk breakdown diode. The third aspect is that of so siting therectifier diodes as to allow charge carriers from them, when conductive,to flow to the adjacent bulk breakdown diode, which may not havetriggered, and which would then be triggered by the supply of chargecarriers from the rectifier diode.

It will be understood that all the diodes may be reversed withoutaltering the operation of the circuit represented by FIG. 4.

Referring to FIG. 2, the second form of semiconductor component forlimiting transient voltages occupies less area than the first form ofthe semiconductor component. The omission of the isolation diffusion ofthe first form of the component provides a first reduction in the arearequired by the component. There is preferential conduction by the"vertical" structures (that is, from the p-type region 201, say, to then⁺ -type region 205, say, of the first bulk breakdown diode-rectifiersdiode pair 200) rather than by the "lateral" structures (that is, fromthe p-type region 213, say, to the n⁺ -type region, say, between thefirst and second bulk breakdown diode-rectifier diode pairs 200 and210). The area required for the rectifier diode is smaller than thatrequired for a bulk breakdown diode because the rectifier diode exhibitsa lower conduction voltage and is required to dissipate less power thanits associated breakdown diode. The bulk breakdown diodes used for thesecond form of the component require a breakdown voltage about twicethat required for the first form of the component (since, in the secondform of the component there is a single bulk breakdown diode setting thevoltage limit for the protected system whereas, in the first form of thecomponent, there are two series-connected bulk breakdown diodes settingthe limit voltage for the protected system). Up to triggering voltagesof about 200 V, the area required for a bulk breakdown diode decreaseswith increasing surge current capacity; since the bulk breakdown diodesrequired for the first and second forms of the component come within the200 V limit, each bulk breakdown diode of the second form of thecomponent requires an area of no more than half the area of either thefirst or third bulk breakdown diode of the first form of the component.

Referring to FIG. 2, in the second form of the component, the n-typeregions represented by the n-type regions 203 and 223 (there is a thirdn-type region in the second bulk breakdown diode but that is not visiblein FIG. 2) are implanted at the same operation, which makes for closematching of the breakdown voltages of the three bulk breakdown diodes.Further, as all of the formations (the n⁺ -type regions) which set thetriggering voltages of the bulk breakdown diodes are on one side of thesemiconductor body the results obtained are more consistent than is thecase where there are voltage-setting formations on both sides of thesemiconductor body.

Referring to FIG.2, the thickness of the n⁺ region 212 can be controlledadjacent to the n⁺ region 211 by varying the depth of the n⁺ region 211and, in particular, the n⁺ region of the bulk breakdown diode of thebulk breakdown diode/rectifier diode pair 210 can be thicker than the n⁺region 212 of the rectifier diode of that pair. An arrangement with athinner n⁺ region 212 provides a rectifer diode with a lower overshoot,on forward conduction, than an arrangement in which the n⁺ region 212has substantially the same thickness as the regions 202 and 222, say.Thickness is measured in the direction of current flow through thesemiconductor component, that is, from the n⁺ region 210 to the p region213. Alternatives for reducing the thickness of the n⁺ region 212 are(i) increasing the depth of the n⁺ region 211, (ii) increasing the depthof the p region 213, or (iii) increasing the depths of both the region211 and the region 213.

Referring to FIG. 6, a unidirectional bulk breakdown diode may berepresented, as shown, by a simplified structure which includes a p-typelayer 50, an n⁻ -type layer 51, an n-type layer 52, a p-type layer 53,and an n⁺ -type layer 54.

Referring to FIG. 7, the simplified structure of FIG. 6 may be redrawn,as shown, by separating the layers 51, 52 and 53, into the regions501-502-503, 521-522-523, and 531-532-533, respectively.

FIG. 8 is an equivalent circuit of FIG. 6 or FIG. 7. Referring to FIG.8, a gradually increasing voltage applied between the emitter electrodesof the transistors is passed on through the emitter-base diode of thePNP transistor 55 and will eventually become large enough to cause thezener diode 58 to conduct current; that current causes the injection ofcharge into the base electrode of the NPN transistor 56, making the NPNtransistor 56 conductive. Once the NPN transistor 56 becomes conductive,regenerative switching takes place with the PNP transistor 55.Switch-off takes place once the available current falls below theholding current for the device, when current shunted by the resistors 59and 60 robs the NPN transistor 56 of base drive and it switches offdegeneratively with the PNP transistor 55.

Referring to FIG. 8, a rapidly increasing voltage applied between theemitters of the transistors is conducted to the base electrode of theNPN transistor 56 and makes that transistor conductive. Conduction, fora time, occurs because the NPN transistor 56 and the zener diode 58 forman "amplified zener", but the PNP transistor 55, under the influence ofthe high drive current taken by the NPN transistor 56, is forced to turnon and aids regenerative turn-on. The device responds in the mannerdescribed because the PNP transistor 55 is designed for a high reverseemitter-base breakdown voltage (so that the device has the capability ofblocking a high reverse voltage) and is, consequently, slow inoperation.

FIG. 10 is a simplified structural representation of a combined bulkbreakdown diode and rectifier diode, connected as shown in FIG. 11,consisting of an n⁺ -type layer 70, a p-type layer 71, an n⁻ -type layer72, an n-type layer 73, a p-type layer 74, and an n⁺ -type layer 75.

FIG. 12 represents the operating characteristic of the combination of abulk breakdown diode 81 and a rectifier diode 80, as shown in FIG. 11.

Referring to FIG. 12, the path OA represents the initial breakdown ofthe bulk breakdown diode, the path AB represents the change to thelow-impedance state, and the path BC represents conduction in thelow-impedance state. The path OD represents forward conduction of therectifier diode.

FIG. 13 represents the operating characteristics of a pair of bulkbreakdown diodes (which need not be identical).

FIG. 18 represents, in plan, an alternative integrated circuitarrangement of the third form of semiconductor component suitable forlimiting transient voltage charges, which is an alternative integratedcircuit arrangement for the component represented by FIG. 4, and FIG. 19represents a sectional view through the integrated circuit representedby FIG. 18 taken along the line Y--Y.

Referring to FIG. 18, the integrated circuit includes respective first,second, and third bulk breakdown diode-rectifier diode pairs 170, 171,and 172. The integrated circuit is shown in FIG. 18 without the contactlayers on the respective upper surfaces of the bulk breakdowndiode-rectifier diode pairs 170, 171, and 172, so that the n⁺ -typeregions 174, 175, and 176 are visible. The integrated circuit includesthree separate p-type regions 177, 178, and 179 and an n⁺ -typeformation 173 which runs along the outer periphery of the integratedcircuit and between the p-type regions 177, 178, and 179.

Referring to FIG. 19, the structure of each of the bulk breakdowndiode-rectifier diode pairs 170, 171, and 172 is substantially the sameas is shown in FIG. 15; in FIG. 18, the p-type region 177, the n⁻ -typeregion 181, and the n⁺ -type region 185 correspond to the respectiveregions 204, 202, and 207 of FIG. 14, and, in FIG. 19, the n⁺ -typeregion 176, the p-type region 179, the n-type region 180, the n⁻ -typeregion 181, and the p-type region 187 correspond to the respectiveregions 205, 204, 203, 202, and 201 of FIG. 14. The regions 177, 181,and 185 shown in FIG. 19 belong to the first bulk breakdowndiode-rectifier diode pair 170, and the regions 176, 179, 180, 181, and187 of FIG. 19 belong to the third bulk breakdown diode-rectifier diodepair 172.

Referring to FIG. 18, the relative orientations of the bulk breakdowndiode-rectifier diode pairs 170, 171 and 172 are such as to increase thetendency of one bulk breakdown diode, when triggered, to trigger theother bulk breakdown diodes. The first bulk breakdown diode-rectifiersdiode 170 has a substantially rectangular shape in the plane of thefigure, while each of the second and third bulk breakdowndiode-rectifier diode pairs 171 and 172 has a substantially square shapein the plane of the figure. The second and third bulk breakdowndiode-rectifiers diode pairs 171 and 172 are positioned side by side andalongside one longer side of the first bulk breakdown diode-rectifierdiode combination 170. The bulk breakdown diodes of the bulk-breakdowndiode rectifier pairs are grouped about the middle of the integratedcircuit, providing a structure in which electrical charge generated inany one of the three bulk breakdown diode-rectifier diode pairs, throughits being triggered, flows quickly to the adjacent bulk breakdowndiodes, causing them both to be triggered. As is shown in FIG. 19, then⁻ -type region 181 is common to all the bulk breakdown diode-rectifierdiode pairs 170, 171, and 172, and charge generated in that region inany one of the bulk-breakdown diode-rectifier diode pairs has arelatively short distance to travel in order to enter each of the otherbulk breakdown diode-rectifier diode pairs. Therefore, the componentrepresented by FIGS. 18 and 19 operates in substantially the waydescribed for the component represented by FIG. 2, but the closegrouping of the bulk breakdown diode-rectifier diode pairs provides acomponent with more rapid mutual triggering than the componentrepresented by FIG. 2.

Referring to FIGS. 18 and 19, two of the bulk breakdown diodes, thecathode regions 175 and 176 of which would be connected respectively tothe terminals 251 and 253 of FIG.4, are side by side and alongside therectifier diode which is in shunt with the third bulk breakdown diode(the third bulk breakdown diode would have its cathode region 174connected to the terminal 252 of FIG. 4). Therefore, the bulk breakdowndiodes which would be connected to the A or Ring (terminal 251) and B orTip (terminal 253) lines of a telephone system are close to therectifier diode which would be connected to the ground line of thetelephone system. In addition, those parts of the cathode regions 175and 176 which are closest to the adjacent rectifier diode and to oneanother have no gaps filled by the adjacent p-type gate region 179,which provides bulk breakdown diodes which are more sensitive at thoseregions than elsewhere. Alternatively, enhanced sensitivity at thoseregions may be obtained by having fewer gaps in those regions, orsmaller gaps in those regions.

The mutual triggering of the bulk breakdown diodes of the componentrepresented by FIGS. 18 and 19 may be further improved by providing suchn-type regions as that represented by the region 180 in FIG. 19 in acentral region only of the component, in order to concentrate thecurrent in the central region of the component. The improvement inmutual triggering may be accompanied by a reduction in the triggeringcurrent of the component and increased voltage overshoot for rapidlyincreasing currents. The tendency of the modified component to allow avoltage overshoot, under the specified conditions, may be reduced by theprovision of a second n-type region which extends over the entire activearea of the component. Each bulk breakdown diode with two n-type regionswould be equivalent to two bulk breakdown diodes, one of the two bulkbreakdown diodes being smaller than the other and having a highersensitivity and a lower triggering voltage than the other bulk breakdowndiode. The bulk breakdown diode with two n-type regions operates asfirst and second bulk breakdown diodes in which the first, smaller bulkbreakdown diode, will trigger first and be the sole current conductoruntil its rate of current rise reaches some set value; that set value ofcurrent corresponds to a voltage overshoot which activates the second,larger bulk breakdown diode.

Referring to FIG. 17, the n-type region 12 which is provided at thejunction between the n⁻ -type region 11 and the p-type region 13 ischaracteristic of a bulk breakdown diode which is a PNPN device. As willbe evident from FIG. 17, the region 11 and the region 12 are of the sameconductivity type, and the region 12 is more heavily doped than theregion 11. The component represented by FIG. 17, being a pair of bulkbreakdown diodes, includes a first n-type region 12 for one of the bulkbreakdown diodes and a second n-type region 16 for the other bulkbreakdown diode.

Referring to FIGS. 17 and 19, the region 180 of FIG. 19 corresponds toeither the region 12 or the region 16 of FIG. 19.

Referring to FIGS. 1, 2, 15, 17 and 19, it is evident that the bulkbreakdown diode is a PNPN diode, that is, a multi-junction diode.

The components represented by FIGS. 3 and 4 will be recognised asstar-connected components. In the case of the component represented byFIG. 3, one of the pair of bulk breakdown diodes forming the arm of thestar connected to the ground line of a system to which the component isconnected, triggers when a bulk breakdown in either of the other arms ofthe star is triggered. A result of the star arrangement is that thecentre of the star goes to ground potential when there is a transientvoltage change causing triggering of the component. That result is notobtainable from a delta-connected component and, for respectivedelta-connected and star-connected components with identical bulkbreakdown diodes, the star-connected component will permit half thetransient voltage change between the supply lines that thedelta-connected component will permit.

The components described above are single chip over-voltage protectorshaving three or more terminals. All the structures described may befabricated in a single chip with three or more terminals. The structuresall operate with vertical currents and have higher current capacitiesthan previously known structures which operate with lateral currents.The components respond to common disturbing voltages on the supply linesto which they are connected by reducing the impulse voltage developedbetween the supply lines. The components may include bulk breakdowndiodes having differing triggering voltages adjusted by different levelsof implantation for the n-type regions. Combinations of bulk breakdowndiode-rectifier diode and bulk breakdown diode pairs may be used in thecomponents. Two forms of the component provide coupling between the bulkbreakdown diodes.

We claim:
 1. A semiconductor component suitable for limiting transientvoltages on the supply lines of a system having at least three supplylines comprising at least three input means for connection to respectiveones of the supply lines, and for each input means, a respectivemulti-junction diode which has a threshold voltage at which it changesfrom a high-impedance state to a low-impedance state, eachmulti-junction diode being connected in the same sense between arespective input means and a common terminal, each multi-junction diodebeing paired with a respective further diode connected in shunt with itand in the opposite sense to it and one pair of the diodes havingcurrent capacities substantially equal to the combined currentcapacities of the other pairs of diodes.
 2. A semiconductor component asclaimed in claim 1 wherein at least one, but not all, of the furtherdiodes is a multi-junction diode and each of the other further diodeshas a single PN junction.
 3. A semiconductor component as claimed inclaim 1 wherein each of the further diodes is a multi-junction diode. 4.A semiconductor component as claimed in claim 1 which is in the form ofa semiconductor body having two substantially parallel major surfaceswherein the electrodes of the diodes lie on the major surfaces, theinput means are a plurality of electrical conductors which make contacton one of the major surfaces with the electrodes of respective pairs ofdiodes, a further electrical conductor which is the common terminalmakes contact on the other of the major surfaces with all of theelectrodes at that surface, and the other pairs of diodes aresymmetrically positioned in relation to the pair of diodes havingcurrent capacities substantially equal to the combined currentcapacities of the other pairs of diodes.
 5. A semiconductor component asclaimed in claim 4 wherein the diodes are alternately a multi-junctiondiode and a further diode both along and across the major surface.
 6. Asemiconductor component as claimed in claim 4 wherein each of thefurther diodes is a multi-junction diode and an isolation regionisolates each multi-junction diode pair electrically from the outermulti-junction diode pairs occupying the semiconductor body.
 7. Asemiconductor component as claimed in claim 4 wherein each of thefurther diodes has a single PN junction and a doped region of thesemiconductor body is common to all the diode pairs.
 8. A semiconductorcomponent as claimed in claim 1 wherein each multi-junction diode has asubstantially PNPN structure and has an inner N region which is alightly doped n-type (n⁻) region which is thicker than a lightly dopedn-type (n⁻) region of each further diode, where the thickness of eachregion is measured in the direction of current flow through thecomponent.
 9. A semiconductor component as claimed in claim 1 whereineach multi-junction diode has a substantially PNPN structure andincludes in the inner N region which is a lightly doped n-type (n⁻)region a buried n-type region of greater impurity concentration than then⁻ -region wherein the buried n-type region is adjacent to the inner Pregion of the PNPN structure.
 10. A semiconductor component as claimedin claim 1 in the form of a semiconductor body which is so structuredthat when one of a first pair of diodes conducts, its electrical chargeis free to flow to and initiate conduction of at least one of anotherpair of diodes.
 11. A semiconductor component as claimed in claim 7wherein the multi-junction diodes are grouped around the middle of thesemiconductor body.
 12. A semiconductor component as claimed in claim 11wherein the multi-junction diodes are so structured as to conduct chargemore readily near the middle of the semiconductor body than elsewhere inthe semiconductor body.
 13. A semiconductor component as claimed inclaim 12 wherein each multi-junction diode is so structured as tooperate as a first multi-junction diode near the middle of thesemiconductor body and as a second multi-junction diode elsewhere in thesemiconductor body, the first multi-junction diode being smaller andmore sensitive than the second multi-junction diode.
 14. A telephonecircuit including a semiconductor component as claimed in claim 1.